The present invention relates to implantable cardiac stimulating devices that provide shock therapy for interrupting cardiac arrhythmias. More particularly, the present invention relates to an improved cardiac stimulating device that includes a system for periodically testing the integrity of electrical leads used to deliver therapeutic shocks to cardiac tissue.
One form of cardiac arrhythmia with serious consequences is ventricular tachycardia (VT). VT is a condition where an abnormally high ventricular heart rate severely affects the ability of the heart to pump blood. VT may result in a loss of consciousness due to a decrease in cardiac output. Sustained episodes of VT are particularly dangerous because they may deteriorate into ventricular fibrillation (VF).
VF is the result of rapid and disordered stimulation of the ventricular myocardium, which prevents the ventricles from contracting in a coordinated fashion. VF is the most life-threatening cardiac arrhythmia--unless cardiac output is quickly restored, the patient may suffer severe physiological consequences, including death.
An increasingly common procedure for treating recurrent VT and VF is to implant a small cardiac stimulating device into the body of the patient. These devices automatically detect episodes of VT and VF and administer therapeutic shocks to cardiac tissue in an attempt to revert the arrhythmias. Cardioversion shocks (typically in the range from about 2 joules to about 5 joules) are often administered to interrupt VT. Defibrillation shocks (typically in the range from about 10 joules to about 30 joules) are administered to revert VF and persistent episodes of VT.
The typical adult sinus rhythm range at rest is between 65 and 85 heart beats per minute (bpm). Generally, rates between 60 and 100 bpm are not a cause for concern. This range is called the sinus rate range. Rates falling outside the sinus rate range are known as arrhythmias. An arrhythmia in which the sinus rate is above 100 bpm is called tachycardia. An arrhythmia in which the sinus rate is below 60 bpm is called bradycardia. Pacing devices can be used to provide artificial cardiac pacing to patients exhibiting bradycardia. However, it is increasingly more common to combine pacing devices with cardioverter/defibrillator devices. This allows a physician to prescribe a single cardiac stimulating device that is capable of administering treatment for all forms of arrhythmias, including tachycardia and fibrillation.
Cardiac stimulating devices deliver electrical pulses through electrical leads connected at or near the patient's heart. The electrical lead system may be prone to degradation that can limit the effectiveness of therapy provided by the implantable cardiac stimulating device. There are several reasons why electrical leads may degrade. For example, electrical leads may bind as they are introduced by a physician into the patient's body, thereby subjecting them to excessive local friction. Also, once implanted, the leads may be subjected to constant pressure and local friction caused by normal bodily movements. If the pressure or friction persists, the lead insulation may deteriorate and the conducting wires may partially or completely fracture. Certain types of damage to these leads may have no initial effect on the operating characteristics of the implantable cardiac stimulating device, and may initially go undetected.
Electrical lead integrity is usually assessed soon after implantation by a radiological examination. However, these examinations may not detect minor damage that later can result in lead degradation. Because of the nature of radiological examinations, it is not practical to frequently examine electrical leads by that method. Electrical testing methods are more commonly used to evaluate lead integrity over the operating lifetime of the implantable cardiac stimulating device.
The impedance of electrical leads used with implantable cardiac stimulating devices typically rises slowly after implantation. The normal impedance of a cardiac defibrillating device lead is approximately 30-55 ohms the time of implantation. Several years after implantation, lead impedance should not be more than 30% greater than the impedance at the time of implantation.
If the lead impedance is uncharacteristically high, a lead fracture is usually indicated. Generally if the lead impedance is above 1,000 ohms, lead fracture is almost certain and if it is above 2,000 ohms, lead fracture is certain. Detecting lead fractures is crucial since lead fractures may prevent delivery of effective therapeutic shocks to cardiac tissue.
Some pacemakers are able to periodically test electrical lead integrity by using frequently delivered pacing pulses as test signals, as described in commonly-assigned U.S. Pat. No. 4,899,750, issued on Feb. 13, 1990, to Christer Ekwall of Spanga, Sweden entitled "Lead Impedance Scanning System For Pacemakers." In a similar manner, electrical lead integrity testing by implantable devices that administer higher-energy therapeutic shocks (i.e., cardioversion and defibrillation shocks) has been accomplished by utilizing lead impedance measurements taken during the most recent shock delivery. These measurements are typically analyzed by a physician at the patient's next follow-up visit after delivery of a shock. However, since higher-energy shocks are much less frequently administered than pacing pulses, lead integrity cannot be evaluated by these devices on a regular basis. Therefore, a significant amount of lead degradation may go unnoticed between therapeutic shocks. If the lead damage that occurs between shocks is too severe, it may prevent the device from reverting the next arrhythmia.
There may also be failures or degradation of a less drastic nature which may be intermittent. Indeed, it is common for electrical problems to start as temporary or intermittent failures. Such failures are virtually impossible to detect when testing is done infrequently.
These problems are compounded when pacemaker and cardioverter/defibrillator capabilities are combined into a single device. The high voltages needed for cardioversion and defibrillation can easily damage or destroy the low voltage circuitry of the device. Thus, the lead integrity testing systems found on the relatively low voltage pacemaker side of the device cannot be easily utilized to test the integrity of leads that are used to deliver high energy shocks because of the potential of harm to the low voltage circuitry from high voltage operations. However, because of the desirability of receiving defibrillation or cardioversion therapy shortly after the onset of the arrhythmia, it is desirable that the leads used to deliver such therapy be physically able to deliver such therapy.
What is needed, therefore, is a system and method for periodically evaluating high voltage electrical lead integrity independent of the delivery of therapeutic shocks. The electrical lead integrity testing system should make data available to the physician at the next follow-up visit regardless of whether a shock had been delivered since the last visit. In addition, because of the desire to administer VT or VF therapy shortly after the onset of an arrhythmia, the electrical lead integrity testing system should be capable of warning the individual that a failure which may prevent delivery of a therapeutic shock has occurred, thus allowing the individual to seek repair of the leads before such a shock is actually needed. Furthermore, the circuitry needed to test the integrity of leads on the cardioverter/defibrillator side of the device must not only be able to withstand such high voltages, but must also guard against the possibility of damage to the low voltage circuitry. Thus, there is a need to electrically isolate the high and low voltage operations of the device while at the same time allowing communication between them. Moreover, it is desirable that the amount of additional hardware added to the device to accomplish both the isolation and high voltage lead integrity testing be minimal because of the limited space available in implantable cardiac stimulating devices.